The present invention relates to an image processing apparatus and, in particular, to an image processing apparatus that binarizes multi-level pixel data and outputs them.
There has previously been known an average density approximation (Information Processing Society of Japan (thesis) Vol. 26, No. 1, 1985) as one of the pseudo-halftone processing methods by which image data is binarized with high precision.
This technique will now be explained briefly. In binarizing input multi-level pixel data, a weighted average density value when an objective pixel is binarized to "1" and a weighted average density value when an objective pixel is binarized to "0" are determined by using binary data in the vicinity of an objective pixel which has already been binarized. The average value of these two weighted average density values is computed. Therefore, the objective pixel is binarized by comparing the computed average value with the value of the objective multi-level pixel. In this method, however, the number of gradations which can be expressed is limited by the magnitude of a weight mask for computing an average density value. Consequently, there arises a problem in that a halftone image cannot be binarized.
Meanwhile, a method has been proposed in which a binary conversion error caused when an objective pixel is binarized is distributed to the next pixel and an overall density is stored so as to improve gradation. In this case, however, since the binary conversion error is distributed to the next pixel adjacent to the objective pixel, the correlation between adjacent pixels becomes lower. For example, when an objective pixel is binarized to "1", since an error caused is negative sign, the adjacent pixel is likely to be binarized to "0". Conversely, when the objective pixel is binarized to "0", since the error caused is positive in sign, the next pixel is likely to be binarized to "1". When the correlation between adjacent pixels is low as described above, if a reduced image is produced by, for example, thinning out half of the binarized data, only "1"s are thinned out or only "0"s are thinned out, and therefore the image after thinning out deteriorates considerably. If an image is displayed on an interlace type display device such as a CRT monitor that performs interlaced scanning every other line, a problem arises, namely, flickering occurs in a screen because the correlation between fields is low.
Another known pseudo-halftone processing includes an error diffusion method. Since a binary conversion error is distributed to adjacent pixels at a maximum ratio, the same problem as above arises.